In a gas turbine engine, ambient air is drawn into a compressor section. Alternate rows of stationary and rotating aerofoil blades are arranged around a common axis, together these accelerate and compress the incoming air. A rotating shaft drives the rotating blades. Compressed air is delivered to a combustor section where it is mixed with fuel and ignited. Ignition causes rapid expansion of the fuel/air mix which is directed in part to propel a body carrying the engine and in another part to drive rotation of a series of turbines arranged downstream of the combustor. The turbines share rotor shafts in common with the rotating blades of the compressor and work, through the shaft, to drive rotation of the compressor blades.
It is well known that the operating efficiency of a gas turbine engine is improved by increasing the operating temperature. The ability to optimise efficiency through increased temperatures is restricted by changes in behaviour of materials used in the engine components at elevated temperatures which, amongst other things, can impact upon the mechanical strength of the blades and rotor disc which carries the blades. This problem is addressed by providing a flow of coolant through and/or over the turbine rotor disc and blades.
It is known to take off a portion of the air output from the compressor (which is not subjected to ignition in the combustor and so is relatively cooler) and feed this to surfaces in the turbine section which are likely to suffer damage from excessive heat. Typically the cooling air is delivered adjacent the rim of the turbine disc and directed to a port which enters the turbine blade body and is distributed through the blade, typically by means of a labyrinth of channels extending through the blade body.
Cooling air from the compressor arrives at the face of the turbine rotor disc and is contained by means of an annular seal-plate aligned co-axially with the turbine rotor a short axial distance from the rotor to provide an annular reservoir of coolant. Small ducts extend from this reservoir to the roots of the blades which contain a labyrinth of cooling channels within their bodies. The air is drawn into the blades and circulates through the labyrinth to cool the blade body.
A seal plate is conventionally secured to both the blades and disc of the rotor. In known arrangements, this is achieved by spigot connections between the plate and the disc at a radially inward portion of the plate and disc and separate, anti-rotation connection adjacent the rim of the plate with each of the blade roots. On an end facing downstream of the coolant flow, the blade roots each have a solid face into which is provided a recess of substantially rectangular cross section. The plate is provided with an array of protrusions, also of substantially rectangular cross section, each sized to fit snugly into a blade root recess. It will be appreciated that manufacturing tolerances for the recesses and protrusions are necessarily tight to ensure a sealing engagement between each plate protrusion and a corresponding blade recess.
The invention provides an alternative plate to blade root connector arrangement which serves the anti-rotation function and provides identifiable benefits to the manufacturer.